This is Doom: The Mercenaries, developed by a modder called DooMero. Basically, it lets you play Doom in a third-person view, controlling either one of five Resident Evil 4 characters: Leon Kennedy, Albert Wesker, Jack Krauser, Ada Wong, or HUNK. The mod is out and playable right now, though its newest version, 4.0, is currently on the way—you can see what it will look like in the video below.

The current release can be downloaded here. To play it, all you need is a copy of Doom and either Zandronum or Skulltag. If you don't feel up to it, however, it's totally okay to just stare at the clip and marvel at what modding is capable of.